FIG. 1A is a schematic circuit diagram illustrating the relationship between a transmitter and a receiver in a current mode according to the prior art. The transmitter Tx comprises a differential pair including two transistors M1 and M2. The two transistors M1 and M2 are controlled according to differential signals DP and DN, respectively. When the transistor M1 is turned on, a current Icm flows through the transistor M1. Consequently, a current with a magnitude Icm/2 flows to a resistor R of the receiver Rx through channels. Under this circumstance, a voltage drop across the resistor R of the receiver Rx is equal to (R×Icm)/2.
FIG. 1B is a schematic circuit diagram illustrating the relationship between a transmitter and a receiver in a voltage mode according to the prior art. When the transistors Mp1 and Mn2 of the transmitter Tx are turned on according to differential signals DP and DN, a current Ivm flows to a resistor 2R of the receiver Rx through two channels. Under this circumstance, a voltage drop across the resistor 2R of the receiver Rx is equal to (2R×Ivm).
If the voltage drop across the resistor R of the receiver Rx of FIG. 1A and the voltage drop across the resistor 2R of the receiver Rx of FIG. 1B are equal, (R×Icm)/2=2R×Ivm. Consequently, Icm=4×Ivm. In other words, for providing the same intensity signal to the receiver Rx, the magnitude of the current provided by the voltage mode transmitter Tx is lower. Consequently, in comparison with the current mode transmitter Tx, the voltage mode transmitter Tx has lower power consumption.
Generally, the channel between the transmitter Tx and the receiver Rx may result in frequency-dependent attenuation. For mitigating the inter-symbol-interference (ISI) resulting from the frequency-dependent attenuation, the signal to be transmitted from the transmitter Tx to the receiver Rx is previously processed by an equalization technique.
FIG. 2A is a schematic timing waveform diagram illustrating the output signals of the transmitter, in which the output signals are not processed by the equalization technique. FIG. 2B is a schematic timing waveform diagram illustrating the output signals of the transmitter, in which the output signals are processed by the equalization technique. Generally, the transmitter Tx and the receiver Rx are in communication with each other through two channels CH1 and CH2. In other words, the channels CH1 and CH2 are substantially a low pass filter. The high-frequency components of the output signals Vo+ and Vo− are largely attenuated by the channels CH1 and CH2. The low frequency components of the output signals Vo+ and Vo− are slightly attenuated by the channels CH1 and CH2.
Please refer to FIG. 2A. Since the output signals Vo+ and Vo− from the transmitter Tx are not processed by the equalization technique, the output signals Vo+ and Vo− have rectangular waveforms. After the output signals Vo+ and Vo− are transmitted through the channels CH1 and CH2, the output signals Vo+ and Vo− are turned into the input signals Vi+ and Vi− of the receiver Rx. Obviously, the high-frequency components of the input signals Vi+ and Vi− are attenuated and suffered from serious distortion.
Please refer to FIG. 2B. Since the output signals Vo+ and Vo− from the transmitter Tx are processed by the equalization technique, high-frequency components of the output signals Vo+ and Vo− are previously emphasized. After the output signals Vo+ and Vo− are transmitted through the channels CH1 and CH2, the output signals Vo+ and Vo− are turned into the input signals Vi+ and Vi− of the receiver Rx. Under this circumstance, the distortions of the input signals Vi+ and Vi− are largely reduced.
FIG. 2C schematically illustrates the definition of a de-emphasis value. According to the equalization technique of the transmitter Tx, a de-emphasis value De is defined as: De=20×log [(X−Y)/(X+Y)], where Y is the amplitude of the emphasized high-frequency component and X is the amplitude of the original high-frequency component.
Recently, a source series termination (SST) voltage mode transmitter is disclosed. The SST voltage mode transmitter is described in IEEE Journal of Solid-State Circuit, Vol. 43, No. 12, December 2008. FIG. 3 is a schematic circuit diagram illustrating a conventional SST voltage mode transmitter.
As shown in FIG. 3, the transmitter (Tx) 300 comprises N SST units 311˜31N and a pre-driver 320. In the N SST units 311˜31N, K SST units 311˜31K are enabled. Moreover, the K SST units 311˜31K are divided into a first portion of X SST units and a second portion of Y SST units, wherein K=X+Y.
In a normal operating situation, the pre-driver 320 controls the X SST units to generate an output signal Vo. For emphasizing the high-frequency component, the pre-driver 320 controls the (X+Y) SST units to generate an output signal Vo. Consequently, the de-emphasis value De is defined as: De=20×log [(X−Y)/(X+Y)].
Since the transmitter (Tx) 300 adjusts the de-emphasis value De according to the characteristics of the channels, it is necessary to previously design a sufficient number of SST units (e.g. N=100) in the transmitter (Tx) 300. Moreover, according to the characteristics of the channels, the X SST units and the Y SST units are respectively enabled to acquire the desired de-emphasis value De. For example, if X=13 and Y=2, the de-emphasis value De is equal to 20×log [(13−2)/(13+2)]. Alternatively, if X=25 and Y=8, the de-emphasis value De is equal to 20×log [(25−8)/(25+8)].
As mentioned above, it is necessary to previously design a sufficient number of SST units in the transmitter (Tx) 300, and it is necessary to enable the K SST units to acquire the desired de-emphasis value De. Obviously, the total of N SST units may occupy much layout area in the circuitry design. Moreover, since (N-K) SST units of the conventional transmitter (Tx) 300 are disabled and unavailable, an unmatched impedance problem occurs.
In other words, after the de-emphasis value De is adjusted, the number of the enabled of SST units of the conventional transmitter (Tx) 300 is changed. Consequently, the output impedance Zo is changed. Under this circumstance, the unmatched impedance between the transmitter Tx and the receiver Rx occurs. For solving this problem, the conventional transmitter Tx has to be additionally equipped with an impedance calibration circuit. Under this circumstance, the circuitry complexity of the transmitter Tx is increased.